1. Field of the Invention
The present invention generally relates to the field of stretch blow molding apparatus for producing plastic containers from parisons, and more particularly concerns stretch blow molding methods and apparatus utilizing cryogenic fluids to cool plastic containers stretch blow molded from parisons.
2. Description of the Prior Art
In biaxially stretch blow molding parisons into plastic containers, a parison may be placed within a blow mold cavity and mounted to a parison engagement assembly, with a stretching rod moved into and along the longitudinal axis of parison, stretching the parison longitudinally. Blow air is introduced into the parison, laterally stretching the parison transverse to the stretching rod. To provide such stretch blow molded containers with heat resistant characteristics, permitting filling with heated fluid, containers may be heat-set to favorably adjust the orientation of the polymers. In order to shorten the operational time required to heat-set stretch blow molded containers, cooling fluids may be introduced within the containers, quickly quenching and heat-setting the container to permit rapid removal from the blow mold.
The introduction of cryogenic liquids through a stretching rod into the interior of a container that has been stretch blow molded from a parison is known in the art, for instance, as disclosed in U.S. Pat. No. 5,182,122. Further, apparatus and method of making a partially crystalline container by stretch blow molding within a hot mold and then injecting a cooling fluid within the blown container is also known, as described in U.S. Pat. No. 4,883,631, and corresponding United Kingdom patent application No. 2,195,287. Introduction of liquid nitrogen within a container for purposes of cooling is disclosed in U.S. Pat. Nos. 5,290,506; 4,375,947 and 4,376,009. The use of U.S. Pat. Nos. 5,290,506; 4,375,947 and 4,376,009. The use of cryogenic gas is also known in extrusion blow molding, as described in U.S. Pat. No. 3,789,093.
Unfortunately, the use of cryogenic fluids with stretch blow molding apparatus may also cause certain components of the apparatus to cool to a temperature where the apparatus becomes inoperative. In particular, cryogenic fluids may lower the temperature of components in contact with blow air to a level where water vapor present in the blow air sublimates onto surfaces of such cryogenically cooled components, causing an accumulation of water ice on the surfaces. With continued accumulation of water ice, the apparatus may be rendered inoperable. In addition, when the temperature of the sealing surfaces of seals present in a stretch blow mold apparatus decreases below a certain level that is characteristic of the materials from which the seals are formed, the seals may fail catastrophically. As a result, there exists a need for methods and apparatus for preventing accumulation of water ice and seal failure during stretch blow molding using cryogenic fluid.
U.S. Pat. No. 5,182,122 has proposed a solution to problems created by utilizing cryogenic fluids which includes using a stretching rod having an inner tube for delivery of a cooling fluid and a concentric outer tube for delivery of a thermally insulating fluid. Despite the availability of such devices, there exist a need in the art for methods and apparatus which permit the introduction of cryogenic fluid within a stretch blow molded container for rapid cooling while preventing accumulation of water ice and maintaining the integrity of the seals, but without the necessity of providing for delivery of a thermally insulating fluid.
In order to aid in the understanding of the present invention, it can be stated in essentially summary form that it is directed to methods and apparatus for stretch blow molding a parison into a container including the introduction of a cryogenic fluid within the container for rapid cooling and providing a source of heat to prevent the cryogenic fluid from causing accumulation of water ice and failure of seals.
More specifically, the present invention includes an apparatus for stretch blow molding a plastic parison into a container, for use with a parison engagement assembly for engaging a parison for stretch blow molding within a blow mold. The apparatus includes a blow mold rod seal assembly having a thermally conducting blow manifold, a thermally conductive blow seal housing, and a thermally conductive base plate mounted to a top plate so that the blow seal housing is sandwiched between and in thermal contact with the blow manifold and the base plate.
A blow manifold defines a first stage blow air inlet port for connection to a source of blow air through a blow air fitting, and further defines a second stage blow air inlet port for connection to the source of blow air. The blow manifold also defines an interior cavity first portion disposed in fluid connection with the first and second stage blow air inlet ports, a chamfered countersink communicating with the interior cavity first portion, and a blow air outlet port.
The blow seal housing includes an upper end and a chamfered lower end, with dimensions of the lower end selected for the blow seal housing to be disposed proximate to the blow manifold with the lower end in mating engagement with the countersink. The blow seal housing defines an interior cavity second portion having a generally cylindrical smaller chamber and a coaxial generally cylindrical larger chamber separated from the smaller chamber by a step. The blow seal housing further defines a first low pressure air supply orifice, disposed through the blow seal housing and communicating with the interior cavity second portion. With the blow seal housing engaged with the blow manifold, the smaller chamber is disposed proximate to the interior cavity first portion.
The base plate is disposed proximate to the blow seal housing and defines an interior cavity third portion having a generally cylindrical first chamber and a generally cylindrical second chamber separated by a second step. The dimensions of the first chamber may be selected to correspond with the exterior dimensions of the upper end of the blow seal housing, so that the base plate and the blow seal housing may be mounted together with the upper end in mating engagement with the first chamber and bearing against a portion of the second step. The second chamber is smaller in radius than the larger chamber, and is disposed proximate to and coaxially aligned with the smaller chamber and the larger chamber. The base plate further defines a circumferential first seal slot at the second chamber, and also defines a second low pressure air supply orifice, disposed through the base plate and communicating with the interior cavity third portion.
The top plate defines a plate opening, and has generally planar plate upper and lower surfaces. The plate opening is disposed above the smaller chamber, the larger chamber, and the second chamber, with the plate lower surface proximate to the base plate.
A piston is provided and includes a piston first end, a piston second end, and a generally cylindrical exterior surface having an exterior surface first portion disposed proximate to the piston first end with diameter slightly smaller than the diameter defined by the smaller chamber of the blow seal housing. The exterior surface also has an exterior surface second portion defining a diameter slightly smaller than the diameter defined by the larger chamber of the blow seal housing and disposed intermediate to the piston first and second ends, and further includes an exterior surface third portion defining a diameter slightly smaller than the diameter defined by the second chamber of the base plate and disposed proximate to the piston second end. A circumferential first shoulder is defined between the exterior surface first and second portions, and a circumferential second shoulder is defined between the exterior surface second and third portions. The exterior surface second portion defines a circumferential second seal slot, and the exterior surface first portion defines circumferential third and fourth seal slots, respectively. The exterior surface third portion also defines a circumferential switch slot. The piston defines a piston passage extending therethrough. The piston passage includes a piston passage first portion separated from a larger piston passage second portion by a piston passage step, and a blow seal step disposed at the exterior surface third portion.
The piston is slidably disposed within the interior cavity second and third portions, with the exterior surface first portion proximate to the smaller chamber, the exterior surface second portion proximate to the larger chamber, and the exterior surface third portion proximate to the plate opening. The piston is thus disposed partially within the interior cavity second portion, the interior cavity third portion, and the plate opening, for sliding movement between a first,lowered position separate from the parison engagement assembly, and a second, raised position in contact with the parison engagement assembly. In the first, lowered position, the piston is disposed so that the piston first end is in contact with the countersink of the blow manifold, the first shoulder is in contact with the step of the blow seal housing, and the piston second end is generally flush with the plate upper surface. In the second, raised position, the second shoulder is in contact with the second step and the piston second end is elevated, projecting above the plate upper surface and in contact with the parison engagement assembly.
A flexible, resilient first O-ring seal is disposed in the first seal slot at the second chamber of the base plate, surrounding and bearing against the exterior surface third portion of the piston, making sealing contact between the piston and the interior cavity third portion. A flexible, resilient second O-ring seal is disposed in the second seal slot, surrounding the exterior surface second portion of the piston, and bearing against the blow seal housing at the larger chamber, making sealing contact between the piston and the interior cavity second portion. In addition, a flexible, resilient third O-ring seal is disposed in the third seal slot, surrounding the exterior surface first portion of the piston, and bearing against the blow seal housing at the smaller chamber, and also making sealing contact between the piston and the interior cavity second portion. The O-ring seals may be formed of a flexible, resilient polymeric material. A U-cup seal is disposed in the fourth seal slot, surrounding the exterior surface first portion of the piston, and bearing against the blow seal housing at the smaller chamber.
Disposed within the piston passage second portion is a rod bearing retainer. A rod bearing chamber is defined through and within the rod bearing retainer and includes a rounded, circumferential interior shoulder. The exterior dimensions of the rod bearing retainer are preferably chosen to correspond with the dimensions of the piston passage second portion, so that the upper surface of the rod bearing retainer is flush with the blow seal step and the rod bearing retainer bears against the piston passage step. Disposed within the rod bearing chamber is a rod bearing. The dimensions of the rod bearing may be selected so that the rod bearing fits between the interior shoulder of the rod bearing retainer and the piston passage step. A blow seal is disposed partially within the blow seal step, bearing against the rod bearing retainer. The blow seal, the rod bearing retainer and the rod bearing may be mounted to the piston using a snap ring disposed in a circumferential snap ring slot defined in the blow seal.
A heater is provided for thermal contact with the blow mold rod seal assembly, whereby heat may be introduced into the blow mold rod seal assembly so that a portion of the O-ring seals and the U-cup seal in contact with the blow mold rod seal assembly may be maintained at a temperature above a predetermined level. The predetermined level is chosen so that the O-ring seals and the U-cup seal maintained at temperatures above the predetermined level retain their sealing properties and do not adhere to surfaces disposed in sliding contact with the seals when a cryogenic fluid is utilized to cool plastic containers blow molded from parisons. The heater may include a plurality of first heater elements for heating the blow manifold, with each first heater element mounted at least partially within one of a plurality of first heater element cavities defined in the blow manifold. The heater further includes a plurality of second heater elements for heating the base plate, each second heater element mounted at least partially within one of a plurality of second heater element cavities defined in the base plate. The first and second heater elements may be electrical resistance heater elements connected to a source of electrical power. The first heater elements provide heat energy to the blow manifold, and the second heater elements provide heat energy to the base plate, whereby the blow seal housing, in thermal contact with the blow manifold and the base plate, receives sufficient heat energy so that at least the outer circumferential portion of the second O-ring seal disposed in sliding and sealing contact with the blow seal housing at the larger chamber, and at least the outer circumferential portions of the third O-ring seal and the U-cup seal in sliding and sealing contact with the blow seal housing at the smaller chamber are maintained at a temperature above the predetermined level. In addition,the first heater elements provide sufficient heat energy whereby at least the inner circumferential portion of the first O-ring seal in sliding and sealing contact with the piston at the exterior surface third portion is maintained at a temperature above the predetermined level.
A piston proximity switch is mounted to a switch mounting plate attached to the base plate so as to be disposed within the plate opening and proximate to the piston. A seal extension housing is mounted below the blow manifold with a coupling extending partially within the blow manifold stepped bore defined in the blow manifold and partially within a seal extension housing first stepped bore defined in a seal extension housing upper end. A seal extension bushing is disposed within a seal extension housing second stepped bore defined in a seal extension housing lower end, and the seal extension bushing and an extension housing U-cup seal are mounted to the seal extension housing with a U-cup retainer and U-cup retainer fasteners. A heater band is mounted surrounding the seal extension housing at the seal extension housing lower end. The extension housing U-cup seal acts to prevent blow air provided to the blow manifold from escaping through the seal extension housing.
A tubular stretching rod having a rod lower end including a threaded portion and a rod upper end is mounted to the blow mold rod seal assembly for vertical sliding movement through the U-cup retainer, the seal extension bushing, the seal extension housing, the coupling, the interior cavity first portion, the piston passage, the rod bearing retainer, the rod bearing, the blow seal, and into the parison engagement assembly. In this way, the stretching rod is disposed to move into and longitudinally stretch a parison coupled to the parison engagement assembly, to facilitate stretch blow molding a parison into a container. Further, the stretching rod acts as a cryogenic fluid tube and thus a cryogenic fluid inlet port at the rod lower end for connection to a source of a cryogenic fluid, and further defines a plurality of cryogenic fluid outlet ports at the rod upper end, whereby a cryogenic fluid may be introduced within a container after stretching and blowing.
The first and second heater elements provide heat to the blow manifold, the base plate, the blow seal housing, the piston and the rod bearing so that portions of the stretching rod disposed within the blow manifold, the base plate, the blow seal housing, the piston and the rod bearing are maintained at a temperature above a preselected level, reducing accumulation of water ice on such portions of the stretching rod that may result from sublimation of water vapor present in blow air. The heater band provides heat to portions of the stretching rod proximate to the seal extension housing lower end sufficient to maintain such portions of the stretching rod at a temperature above the preselected level, reducing accumulation of water ice on the stretching rod from sublimation of water vapor present in ambient air.
Vertical movement of the stretching rod is accomplished using a rodless air cylinder assembly including a cylinder support mounted to a mounting plate, and an air cylinder vertically movable with respect to the cylinder support using air supplied though a flow control orifice from a source of compressed air. A stretching rod tooling, having a stretching rod tooling lower surface, is mounted to and beneath the blow manifold. A stop rod is mounted to the air cylinder, and a stretching rod bumper pad is mounted to the stop rod, for contact with the stretching rod tooling lower surface when the air cylinder moves vertically upward. A stretching rod switch mounting bracket is attached to the stretching rod tooling, and supports a cylinder proximity switch, which is electrically connected to the air cylinder assembly. The air cylinder assembly is also electrically connected to the piston proximity switch.
The cylinder assembly may be coupled to a pair of stretching rods using a stretching rod plate mounted to the air cylinder, with each stretching rod for use with one of a pair of stretch blow molding stations. A brace is attached to the stretching rod plate and defines a brace aperture through which the check valve is mounted. A T-fitting is attached below and in fluid connection with the check valve, and a pair of opposing, generally U-shaped tubes are connected to the T-fitting. Each U-shaped tube is fluidly connected to a stretching rod. Further, each stretching rod is adjustably mounted to stretching rod plate with a tapped stretching rod adjustment bracket attached to stretching rod plate, whereby the threaded portion of each stretching rod is threadably engaged with the stretching rod adjustment bracket. The check valve is fluidly connected to a supply tube through which cryogenic fluid, such as liquid nitrogen, may be supplied to the present invention.
Vertical sliding movement of the piston within the blow seal housing, the base plate, and the top plate is controlled by the introduction of low pressure air into the interior cavity second portion of the blow seal housing through the first low pressure air supply orifice, and into the interior cavity third portion of the base plate through the second low pressure air supply orifice. In the first, lowered position, the piston is disposed with the first shoulder in contact with the step of the blow seal housing, and an upper volume is defined within the larger chamber proximate to the upper end of the blow seal housing. With the piston in the lowered position, the upper volume communicates with the second low pressure air supply orifice and the piston proximity switch is disposed in contact with the exterior surface third portion of the piston, outside of the switch slot. In the second, raised position, the piston is disposed with the second shoulder in contact with the second step of the base plate, and a lower volume is defined within the smaller chamber proximate to the lower end of the blow seal housing. With the piston in the raised position, the lower volume communicates with the first low pressure air supply orifice and the piston proximity switch is disposed in engagement with the switch slot.
In use, low pressure air supplied to the lower volume through the first low pressure air supply orifice raises the piston to the second, raised position, while supply of low pressure air to the upper volume through the second low pressure air supply orifice exerts downward pressure to move the piston to the lowered position. The piston proximity switch acts to signal the position of the piston, indicating whether the piston is in the lowered position or not in the lowered position. The first and second O-ring seals act to contain supplied air within the upper volume, and the second and third O-ring seals act to contain supplied air within the lower volume.
With the piston in the raised position, the piston proximity switch moves into the switch slot and activates the air cylinder assembly, whereby the air cylinder moves vertically upward along the cylinder support. Upward movement of the air cylinder is limited by contact of the stretching rod bumper pad with stretching rod tooling lower surface, triggering the cylinder proximity switch to stop movement of the air cylinder.
The stretching rod slides vertically upward through the U-cup retainer, the seal extension bushing, the seal extension housing, the coupling, the interior cavity first portion, the piston passage, the rod bearing retainer, the rod bearing, the blow seal, and the parison engagement assembly, and into the interior of a parison mounted to the parison engagement assembly. By selecting the length of the stop rod, vertical movement of the stretching rod into a parison may be selected to stretch the parison to form a container of a desired size. In addition, fine adjustment of vertical sliding movement of the stretching rod may be made by rotating and threadably advancing the stretching rod with respect to the stretching rod adjustment bracket.
With the piston in the raised position, blow air may be supplied to the present invention, entering the blow manifold through the first and second blow air inlet ports, into the interior cavity first portion, and also into the lower volume thereby providing additional upward pressure against the piston first end. The U-cup seal provides slidable, sealing contact between the piston and the interior cavity second portion to prevent blow air from entering the larger chamber of blow seal housing. Due to upward pressure on the piston from blow air, the blow seal is urged upward against the parison engagement assembly with sufficient force to form a seal permitting blow air to enter a parison engaged therewith. Blow air flows through the interior cavity first portion of the blow manifold, external to the stretching rod, and into the piston passage, the rod bearing retainer, the rod bearing seal, and the blow seal, so that blow air may pass through the parison engagement assembly for blowing a parison into a container.
After stretching and blowing a parison into a container, high pressure blow air is released from the blow air outlet port so that blow air no longer exerts upward pressure on the piston at the piston first end. Low pressure air is then supplied to the upper volume through the second low pressure air supply orifice, exerting downward pressure on the piston at the second shoulder, causing the piston to move downward to the lowered position.
After formation by stretching and blowing from a parison, a container has a relatively high temperature, and such containers may be advantageously cooled by supplying a cryogenic fluid to the interior of the container, cooling and heat-setting the container from within. The present invention provides for such cooling by introducing cryogenic fluid within a container through the supply tube, the check valve, the U-shaped tube, and into the stretching rod through the cryogenic fluid inlet port. Cryogenic fluid then emerges from the stretching rod at the cryogenic fluid outlet ports, within the container, and acts to quickly quench and heat-set the container to permit rapid removal from the blow mold. The cryogenic fluid also cools the stretching rod, the piston, the blow manifold, the blow seal housing, and the base plate, along with the O-ring seals and the U-cup seal.
Cooling of the stretching rod to a temperature below the preselected level may cause water ice to accumulate thereon, with water ice forming from sublimation of water vapor present in blow air flowing around the stretching rod and through the blow manifold, the piston, and the rod bearing. With continued accumulation of water ice, the stretching rod may be prevented from sliding through the blow manifold, the piston, and/or the rod bearing. To prevent this undesirable cooling of the stretching rod, the first and second heater elements heat the blow manifold, the base plate, the blow seal housing, the piston and the rod bearing so that portions of the stretching rod within the blow manifold, the base plate, the blow seal housing, the piston and the rod bearing are maintained at a temperature above the preselected level. In addition, the heater band is provided to reduce accumulation of water ice from ambient air on portions of the stretching rod proximate to the seal extension housing lower end which might prevent sliding movement of the stretching rod through the seal extension housing. The heater band heats portions of the stretching rod to a temperature above the preselected level, eliminating accumulation of water ice resulting from sublimation of water vapor present in ambient air.
Cooling of the O-ring seals and the U-cup seal to a temperature below a predetermined level may cause seal failure and/or adhesion to surface disposed in sliding contact with the seals. To prevent cooling of the sliding contact surfaces of the O-ring seals and the U-cup seal, the first heater elements warm the blow manifold and the second heater elements provide heat energy to the base plate, whereby the blow seal housing is warmed so that at least the outer circumferential sliding contact portions of the first and second O-ring seals and the U-cup seal are maintained at a temperature above the predetermined level. Correspondingly, the first heater elements also warm at least the inner circumferential sliding contact portion of the first O-ring seal to a temperature above the predetermined level.